[AMDGPU] Fix access beyond the end of the basic block in execMayBeModifiedBeforeAnyUse.

I was wrong in thinking that MRI.use_instructions return unique instructions and mislead Jay in his previous patch D64393.

First loop counted more instructions than it was in reality and the second loop went beyond the basic block with that counter.

I used Jay's previous code that relied on MRI.use_operands to constrain the number of instructions to check among.
modifiesRegister is inlined to reduce the number of passes over instruction operands and added assert on BB end boundary.

Differential Revision: https://reviews.llvm.org/D89386